The invention relates to an electronic component with a semiconductor composite structure in which a doped semiconductor layer is in proximity to an undoped diamond layer.
German Patent Document DE-P 44 15 600.6 (not previously published) discloses a hetero-epitaxial composite structure in which an undoped diamond layer deposited by chemical vapor deposition (CVD) is provided with charge carriers by an adjacent doped semiconductor layer. Such introduction of charge carriers into the undoped diamond layer is achieved by the fact that a band discontinuity exists at the transition between the semiconductor layer and the diamond layer, so that charge carriers from the doped semiconductor layer pass into the undoped diamond layer. In this manner, diamond coatings can effectively be n-doped, especially with the aid of n-doped C-BN (cubic boronitride), because due to the band discontinuity at the middle boundary surface between the diamond coating and the n-doped C-BN layer, charge carriers from the C-BN layer flow into the diamond layer. It would of course be desirable to further increase the mobility of the charge carriers in the diamond layer.
Accordingly, an object of the present invention is to provide a semiconductor composite structure of the type described above, which provides improved mobility of charge carriers in the undoped diamond layer.
This and other objects and advantages are achieved by semiconductor composite structure, in which the undoped diamond layer has a thickness in a range of from 0.001 to 0.5 .mu.m (preferably less than 0.1 .mu.m, and ideally less than 0.05 .mu.m), and has a band discontinuity at the boundary surface on both sides thereof. Due to the band discontinuity that is necessarily present at the boundary surface between the coatings (that is, between the semiconductor coating and the diamond coating), charge carriers generated optically and/or thermally, which are normally present in the doped coating(s), enter into the diamond layer's valence and/or conductivity band. The latter thus forms a "potential well" with quantizing for these charge carriers, as shown, for example, in FIG. 3. On account of the quantizing effects with respect to the charge carriers, it is possible with the composite structure of the invention to produce fast semiconductor components.
Due to the thinness of the undoped diamond layer, as discussed in greater detail hereinafter, the course of the band edge of the composite structure forms a so-called unidimensional potential well for the charge carriers that have flowed down into the diamond coating, at the interface with the adjacent layer or layers that dope the diamond layer. The "walls" of the potential well, formed by the band discontinuities (that is, the sharp increase in the energy level of the conduction band in the diamond layer at its interfaces with the two adjacent layers), lock in the charge carriers. Since the charge carriers in the diamond layer especially are able to assume only one energy state, at least discrete energy state, the scatter of the charge carriers is a least reduced, so that their mobility is increased.
For better comprehension, this mechanism will be described below by way of example with the aid of one especially useful C-BN/diamond/C-BN layer structure, in which the two n-doped C-BN layers are deposited adjacent an undoped diamond layer.
The C-BN layers which are disposed on both sides of the diamond layer, have a greater band energy gap than does the diamond layer. As the course of the band edge is formed, the band boundaries level out according to their particular Fermi energy level. Since the Fermi energy level of the C-BN layer is nearer to the conductivity band than the valence band of the undoped C-BN layer due to the n-doping, and since the band gap of the diamond layer is smaller than that of the C-BN layer, the result is a band discontinuity at the transition from the diamond layer to each C-BN layer. As a result, at least some of the charge carriers flow out of the C-BN layer into the diamond layer.
Since, according to the invention, the band edge in the area of the diamond layer has a shape that is at least similar to a potential well for the charge carriers in quantum physics, the energy levels of the charge carriers within the potential well are far enough apart, so that in the ideal case, only one state is possible in quantum mechanics for the charge carriers in the potential well. Due to this circumstance the probability of collision is low, so that the mobility of the charge carriers in the diamond layer is increased.
Furthermore, the probability of scatter is very low due to the suggested type of doping, since the doping atoms are spatially separated from the charge carriers situated in the potential well.
Furthermore, because the diamond layers of semiconductor composite structures of this type need not be doped, the associated problems of reproducibility, additional contamination, disturbance points for displacements and the like are at least decreased. Thus, ionized charge carrier bodies cannot act, or act only to a small extent, as scattering centers for the charge carriers in the active diamond layer. It has further proven to be advantageous that in semiconductor composite structures of this type, n-doping, as well as p-doping, is possible in d simple and inexpensive manner.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.